Turn on your pool filter. Check your saltwater generator: Most will have a low-salt indicator that alerts you when salt levels are too low. Some will have a digital output that gives you the exact salt reading. If you have one of these, you do not need a testing kit.
Purchase a saltwater testing kit or strips at your local pool supply retailer. Check the expiration date on the testing strips. Expired strips will not give an accurate reading.
Dip the testing strips into the water, and follow the manufacturer's instructions, as they vary. Consult the color chart to obtain the salt level in the water.